1. Field of the Invention
The present invention relates to a grinding method in which an acoustic energy sensor detects a vibration signal emitted from the contact zone between a grinding wheel and a workpiece during grinding. Movement of a work table, during grinding of the workpiece, is reversed based on the change of the vibration signal. Another non-limiting embodiment of the present invention relates to a grinding apparatus for grinding a workpiece mounted on a work table which is reversed based on the change of a time-series vibration signal detected by the acoustic energy sensor.
According to the grinding method of the invention, since the reverse position of the work table is determined by the vibration signal generated by the acoustic energy (vibration) sensor according to the contour of the workpiece during grinding, when the workpiece has a shape such as a disk, a star shape, an L shape, an ellipse, a semicircle or a long shape, a reversely reciprocating path of travel of the work table is shortened, and grinding time can be greatly reduced.
2. Discussion of the Background
Various types of grinding apparatuses are known. For example, JP-A-2000-263436 shows a surface grinding apparatus in which a workpiece placed on a work table which can reciprocate in the horizontal direction is moved relatively to a grinding wheel provided at a wheel head. The wheel can move up and down in the vertical direction, and the work is subjected to surface grinding.
A surface grinding apparatus 1 shown in FIG. 4 and FIG. 5, has a workpiece 2, a grinding wheel 3, a work table 4 which can reciprocate in the horizontal direction (X-axis direction), a working bench part 5, an electromagnetic chuck 6, a saddle 7 which can reciprocate in the forward/reverse direction (Z-axis direction), an operation panel 8, a grinding wheel cutting start point position detection On-Off switch 8a, a grinding wheel vertical cutting manual pulse generator button 8b, a column 9, a grinding wheel spindle 10, a wheel head 11, a lifting/lowering unit 12 to move the wheel head 11 in the vertical direction (Y-axis direction), a motor 13, an engaging body 14, a ball screw shaft 15, a bearing 16, a safety protection cover 17, an NC control device (CPU) 44 and a grinding liquid supply nozzle 30.
The rotating grinding wheel 3 is lowered to the workpiece 2 on the work table 4 reciprocating in the horizontal direction. The grinding wheel makes a cut in the workpiece, and the work table and the grinding wheel are relatively moved so that grinding is performed. At this time, a grinding liquid is supplied from the nozzle 30 to a working point where the workpiece and the grinding wheel come in contact with each other. A hydraulic drive, combination drive of a ball screw and a servo motor, linear motor drive or the like is used for the horizontal movement of the work table 4. A table reversing mechanism uses dogs (hook-like devices) fixed to both left and right sides of the table, and limit switches fixed to a bed and having an interval narrower than the interval of the dogs.
FIG. 6 shows a grinding apparatus 1 in which a work table is hydraulically operated. Dogs and limit switches are used as a table reversing mechanism. The grinding apparatus 1 shown in FIG. 6 has a grinding wheel 3, a work table 4, a column 9 and an NC control device (CPU) 44 having a storage part (RAM) in which a program is written and a recording part (ROM) in which data and the like are recorded. Reference numeral 20 denotes a bed; 21, dogs; 22, limit switches; 23, a hydraulic drive part; 24, a step motor; 25, a hydraulic circuit; 26, a table speed adjustment throttle valve; 27, an input port; 28, an output port; and 29, a motor drive circuit.
The hydraulic drive part 23 is driven by the hydraulic circuit 25 and moves the work table 4 in the horizontal direction. In the horizontal reverse operation, the dog 21 operates the limit switch 22. When the work table reaches the left end, the right limit switch 22 faces the right dog 21 and is turned on, and the signal of the right limit signal 22 is inputted as a reverse detection signal to the control device 44. Accordingly, a direction switchover valve (not shown) of the hydraulic circuit 25 is operated to change (reverse) the operation direction of the hydraulic drive part 23, that is, the operation direction of the work table 4. The work table starts to move in the right direction and the right dog 21 separates from the right limit switch 22.
When the work table 4 reaches the right end, the left limit switch 22 faces the left dog 21 and is turned on, and the signal of the left limit switch 22 is inputted as the reverse detection signal to the NC control device 44. Accordingly, the direction changeover valve (not shown) of the hydraulic circuit 25 is operated to change (reverse) the operation direction of the hydraulic drive part 23, and thus, the operation direction of the table 4. The table starts to move in the left direction, and the left dog 21 separates from the left limit switch 22. Such operation is alternately continuously performed.
In this conventional table reversing mechanism, because the reverse positions at the left and right of the work table are determined using the dogs and the limit switches, the movement distance of one reciprocation of the work table during grinding of the workpiece is always constant. Thus, when the workpiece has a long body such as a rectangular parallelepiped or a cube and is placed in parallel to the chuck and grinding is performed, shortening of the reciprocating movement distance of the work table is difficult to achieve. However, when the workpiece shape is an irregular shape such as a disk, an L shape, a star shape, an ellipse or a semicircle, or when a long body is placed obliquely to the chuck and grinding the workpiece is performed, there is a rather long empty movement in the horizontal reciprocal movement in which the workpiece is not machined. That is, since the reciprocal movement width of the work table is determined in the light of the longest length of the work, when a long workpiece is machined in one reciprocation of the table, a large fraction of the motion of the grinding wheel is effectively used. However, when a short workpiece is machined, there is much waste in the reversely reciprocating distance of the work table. In other words, the grinding table is inefficient at grinding short or irregularly shaped workpieces because the stroke of the grinding wheel may be too large.
As a method of reduction of net machining time for the grinding process, JP-A-2002-370165 shows a grinding method using a hydrophone in which the total reciprocating movement distance of the work table during grinding can be shortened. With this method, it is unnecessary to input the reverse position of the work table from an operation panel each time. A sound pressure signal received by the hydrophone is made to pass through a band-pass filter. A basic frequency is extracted from the sound pressure signal having passed through this band-pass filter. A time when a value (Pi) of the sound pressure signal at the basic frequency reaches a pre-set sound pressure signal (P0) at the basic frequency (Pi=P0) is determined to be a table reverse timing, and reversing of the work table is performed.
The reversing method of the work table using the hydrophone has a benefit in that the total reciprocating movement distance of the work table becomes shorter than that of the reversing method of the work table using the dogs, and grinding time is reduced. However, since the hydrophone receives the sound pressure signal transmitted through the grinding wave, the threshold of the sound pressure signal (Po) at the basic frequency is set to be low to assure safety. Thus, the work table motion is reversed at a position where it is 100 to 200 mm away from a position where the grinding wheel separates from the surface of the workpiece.